The reaction of aromatic dianhydrides such as benzophenone tetracarboxylic dianhydride (BTDA) with aromatic diamines to produce a polyimide acid which can then be heated to produce a polyimide is well known in the art. Such polyimides are useful as adhesives, molding powders and films having excellent chemical resistance and thermal resistivity properties. U.S. Pat. No. 4,065,345 to Donald J. Progar, et. al. describes a method for preparing an adhesion solution of a polyimide acid by reacting an aromatic dianhydride with an aromatic diamine in the presence of certain specified ether solvents including diglyme and other aliphatic acyclic ethers. Progar, et al. teach in column I, lines 57 et seq., that "in certain instances, a highly viscous polyamicacid intermediate polymer precipitates from the ether solvents. The addition of very small amounts of water or alcohol generally leads to redissolution of a polyamic-acid to give a viscous polymer solution."
We confirm the above findings when diglyme, a preferred solvent due to availability and boiling point, is used. The two-step technique of Progar, et al. suffers from the fact that it is a two-step process but more particularly because the precipitated polymer severely limits the type and size of equipment which can be employed commercially. Progar, et al. in their Example II (columns 5 and 6), which uses diglyme as the solvent, employs a household blender to perform the experiment. The normal paddle-stirred devices cannot be employed as the precipitated polymer tends to wrap itself around the paddle blades resulting in potential damage, stoppage and other problems such as an increased energy usage. All of the above problems are overcome using the method of this invention.